1. Field of the Invention
This invention relates to a crosslinked aromatic polymer electrolyte membrane which is suitable for use in a polymer electrolyte fuel cell, and which has low water uptake, high proton conductivity, low methanol permeability, high chemical stability, and excellent mechanical characteristics; and a method for producing the crosslinked aromatic polymer electrolyte membrane.
2. Description of the Related Art
A fuel cell using a polymer electrolyte membrane is operated at a temperature of as low as 150° C., and has a high power efficiency and a high energy density. Thus, such a fuel cell is expected to serve as a power source for mobile instruments, a power source for cogeneration stationary systems, or a power source for fuel cell vehicles (automobiles), which utilizes methanol, hydrogen or the like as a fuel.
In connection with the fuel cell, important component technologies on polymer electrolyte membranes, electrocatalysts, gas-diffusion electrodes, and membrane-electrode assemblies are existent. Development of a polymer electrolyte membrane having excellent characteristics for use in the fuel cell is one of the most important technologies.
In the polymer electrolyte fuel cell, the polymer electrolyte membrane acts as an “electrolyte” for conducting hydrogen ions (protons), and also acts as a “separator” for preventing direct mixing of hydrogen or methanol, as a fuel, with oxygen. The polymer electrolyte membrane is required to have high proton conductivity; excellent chemical stability ensuring long-term durability, especially, resistance to hydroxide radicals becoming a main cause of membrane deterioration (i.e., chemical stability); long-term thermal durability at the operating temperature of the cell, or at even higher temperatures; and constant and high water retention properties of the membrane for keeping proton conductivity high. To play the role of the separator, the polymer electrolyte membrane is required to be excellent in the mechanical strength and dimensional stability, and to have low permeability to hydrogen, methanol and oxygen.
A perfluorosulfonic polymer electrolyte membrane “Nafion (registered trademark of DuPont)” developed by DuPont, for example, has generally been used as the electrolyte membrane for the polymer electrolyte fuel cell. Perfluorinated polymer electrolyte membranes of the related art, such as Nafion, are excellent in chemical durability and stability. However, their water retention properties are insufficient at high temperatures and low humidity. Thus, the drying of the ion exchange membranes occurs, resulting in decreased proton conductivity. They are also disadvantageous in that when methanol is used as a fuel, swelling of the membrane or crossover of methanol takes place.
They have also been defective in that their mechanical characteristics under operating conditions involving temperatures exceeding 100° C., required for an automobile power source, markedly decline. Furthermore, the production of the perfluorinated polymer electrolyte membranes starts with the synthesis of fluorine-based monomers. Thus, the manufacturing process is so complex that a high cost is entailed. These have been a great impediment to the commercialization of these polymer electrolyte membranes-based fuel cells as power sources for stationary cogeneration systems or power sources for fuel cell vehicles.
Under these circumstances, the development of a low-cost polymer electrolyte membrane replacing the perfluorinated polymer electrolyte membrane has been energetically carried out. For example, attempts have been made to prepare partially fluorinated polymer electrolyte membranes by introducing styrene monomers into fluoropolymer films, such as polytetrafluoroethylene (PTFE), poly(vinylidene fluoride) (PVDF), and ethylene-tetrafluoroethylene copolymer (ETFE), by graft polymerization, and then sulfonating the graft polymers (see, for example, JP-A-2001-348439 and JP-A-2004-246376).
However, the fluoropolymer films have a low glass transition temperature, so that their mechanical strength at high temperatures of 100° C. or higher considerably declines. When a high electric current is flowed through the electrolyte membrane for a long time, moreover, the sulfonic acid groups introduced into the polystyrene graft chains become detached, resulting in the marked lowering of the proton conductivity of the electrolyte membrane. There is also the defect that crossover of hydrogen, as the fuel, or oxygen occurs.
On the other hand, an aromatic polymer electrolyte membrane has been proposed as a low-cost hydrocarbon-based polymer electrolyte membrane (see, for example, U.S. Pat. No. 5,403,675). Since the aromatic polymer electrolyte membrane has excellent mechanical strength at high temperatures and low fuel permeability to methanol, hydrogen, oxygen or the like, its use at high temperatures is expected.
The aromatic polymer electrolyte membrane is prepared by dissolving an aromatic polymer material, typified by an engineering plastic, in a sulfonating solution such as concentrated sulfuric acid or chlorosulfonic acid to sulfonate the aromatic polymer, and then forming a solution of the sulfonated aromatic polymer into a membrane by casting (see, for example, JP-T-11-502245 and JP-A-06-049202).
The aromatic polymer electrolyte membrane is also obtained by the polymerization of an aromatic monomer having sulfonic acid groups bound thereto, and then forming the resulting polymer into a membrane (See, for example, JP-A-2004-288497, JP-A-2004-346163, and JP-A-2006-12791).
The aromatic polymer electrolyte membrane has excellent characteristics at high temperatures, so that its use at high temperatures is expected. However, the methods for preparing the aromatic polymer electrolyte membranes disclosed in JP-T-11-502245, JP-A-06-049202, JP-A-2004-288497, JP-A-2004-346163, and JP-A-2006-12791 use large amounts of strong acids for the purpose of dissolving the aromatic polymer materials, and thus use large amounts of diluting water in order to precipitate the sulfonated materials. As noted here, these methods require complicated steps. Moreover, the membrane-forming process by casting needs large amounts of organic solvents.
The electrolyte membranes prepared as above have no crosslinked structure. If the degree of sulfonation is high, or the temperature is heightened, therefore, problems occur, such as dissolution in water, or considerable dimensional changes or marked decreases in strength, due to absorption of water. As noted here, the electrolyte membranes do not possess mechanical strength which enables the shape of the electrolyte membrane to be maintained under the cell operating conditions.
Furthermore, the sulfonic acid groups exist randomly in the aromatic polymer chains, thus resulting in unclear separation between a hydrophobic layer for maintaining mechanical strength and an electrolyte layer in charge of proton conduction. Hence, proton conductivity, fuel impermeability, and chemical stability are insufficient.
The present invention has been accomplished in the light of the above-described problems. It is an object of the invention to provide an aromatic polymer electrolyte membrane which does not cause a problem, such as dissolution in water, or a considerable dimensional change or a marked decrease in strength, due to absorption of water, which possesses mechanical strength enabling the shape of the electrolyte membrane to be maintained under the cell operating conditions, and which is sufficient in proton conductivity, fuel impermeability, and chemical stability.
It is another object of the invention to provide a method for producing the aromatic polymer electrolyte membrane, which does not need complicated steps, can markedly reduce the cost of production, and obviates the need for a membrane-forming step by casting.